This invention relates to a two-stage process, the first stage of which comprises upgrading either light gas, FCC gas, and/or, light naphtha, boiling range 175.degree. C. (347.degree. F.) to 240.degree. C. (464.degree. F.), either of which contains at least 10 percent by weight (% by wt) olefins, to intermediate range hydrocarbons boiling in the range from 50.degree. C. to 204.degree. C. (125.degree. F.-400.degree. F.) ("gasoline") in a primary reactor termed a MOG (for "Mobil Olefin to Gasoline") reactor.
The MOG primary reactor may be operated as either a moving bed, fixed, riser type or fluid bed, the last being the embodiment in which it is preferably operated, for economic reasons. This MOG primary reactor is operated at relatively low weight hourly space velocity ("WHSV", it being understood that WHSV signifies pounds of olefins fed per pound of zeolite per hour) but otherwise under process conditions generally within the ranges specified for those used in a process described in our aforesaid copending '926 application, except that we now operate the MOG reactor to produce higher conversion to gasoline, and at least 1 part distillate for 10 parts gasoline, in an effluent substantially free of aromatics (that is, less than about 3 mol percent), which effluent now contains slightly more paraffins than in our '926 process. Further, the effluent in this invention is condensed and fractionated under conditions different from those in our '926 process so that we now avoid sending C.sub.10 + (C.sub.10 and heavier) components to the secondary reactor thus providing a tailored, olefin-rich C.sub.5 + feed, substantially free of distillate, to a secondary reactor in which the feed is converted either to distillate, or to lubes depending upon the particular preselected mode in which the secondary reactor is operated.
"Distillate" refers to hydrocarbons boiling in the range from 130.degree. C. to 343.degree. C. (266.degree. F.-650.degree. F.); "lubes" refers to hydrocarbons boiling above 343.degree. C. (650.degree. F.). The secondary reactor is termed a MODL (for "Mobil Olefin to Distillate and/or Lubes") reactor, and is referred to as such (i.e. "MODL") when reference is made to its operation either to make, predominantly distillate (distillate mode operation), or to make, predominantly lubes (lube mode operation). When the secondary reactor is specifically being used to make a major proportion by wt of distillate it will be referred to herein as a "MOD" reactor operating in a distillate mode; and, when this reactor is specifically used to make a major proportion by wt of lubes it will be referred to as a "MOL" reactor operating in a lubes mode. The MODL reactor may, under certain circumstances, also be operated to produce an effluent ("MODL effluent") which contains a larger proportion by wt of gasoline than is present when the reactor is operated in the distillate mode. Under such operating conditions the reactor is referred to as a "MOGD" (for "Mobil Olefin to Gasoline & Distillate") reactor.
The specific embodiments of this invention derive from operating the MOG for maximum conversion to gasoline, which operation also produces a specified minimum fraction of distillate. Tailoring the MOG effluent results in a surprisingly effective combination of conventional unit operations which permit either semicontinuous operation of our process using a single fixed bed MODL reactor; or, continuous operation, whether with plural fixed beds, or a single fluid bed MODL reactor. In this "maximum conversion" operation of the MOG reactor, an exceptionally high conversion of light gas (or, FCC gas), or light naphtha to olefins is obtained at below about WHSV=10 hr.sup.-1. The relatively high proportion of distillate formed allows recovery of a significant amount of distillate upstream of the MODL reactor and to tailor the MOG effluent to provide an olefin-rich C.sub.5.sup.+ feedstream substantially free of C.sub.10.sup.+ components to the MODL reactor. But for this tailored olefin feedstream we would not have the unexpectedly high oligomerization of olefins in the MODL reactor along with beneficial processing flexibility and savings in the costs of operation.
Though the general operating conditions of pressure and temperature for both the MOG and the MODL reactors have been disclosed in the prior art, the particular operating conditions of the MOG reactor which produce the substantial distillate content in the MOG effluent at maximum conversion to gasoline, were never recognized. Under the circumstances, considerations relating to the desirablity of separating the distillate from the MOG effluent, or the consequences of doing so, never arose. Nor were the particular operating conditions known for operating a MODL reactor with a tailored C.sub.5 -C.sub.9 feed which produces more distillate than gasoline.
As will be explained in greater detail hereinafter, the specific improvements of providing a debutanizer instead of a deethanizer (used in our '926 application), and placing a gasoline/distillate splitter ("G/D splitter") or a high temperature separator ("HTS") before the MOD reactor in the distillate mode, dramatically reduces the flow to the MOD reactor, decreases the heat of reaction the reactor must cope with, and thus further helps obtain the high conversion in the MOD reactor.
Developments in fluid-bed and fixed bed catalytic processes using a wide variety of zeolite catalysts have spurred interest in commercializing the conversion of olefinic feedstocks to C.sub.5.sup.+ hydrocarbons including gasoline, diesel fuel, lubes, etc. In addition to the discovery that the intrinsic oligomerization reactions are promoted by ZSM-5 type zeolite catalysts, several discoveries relating to implementing the reactions in an apt reactor environment, have contributed to the commercial success of current industrial processes. These are environmentally acceptable processes for utilizing feedstocks containing lower olefins, especially C.sub.3 -C.sub.5 alkenes, though a significant quantity, up to 40% ethylene, along with olefins and paraffins heavier than C.sub.5 may also be present. A predominantly olefinic light gas containing more than 50% by wt, and preferably more than 60%, of combined propene and butenes, is a particularly well-suited feed to oligomerization reactors using a ZSM-5 type catalyst.
In our MOG+MODL combination of staged reactors, only the olefins in the light gas are converted to gasoline in the primary stage fluid-bed MOG reactor, operating with a relatively low activity (alpha) catalyst, at WHSV&lt;10 hr.sup.-1, and moderate pressure and temperature ("low severity" conditions referred to herein as "easy").
When our process operates in the distillate mode, in the first stage, the MOG reactor produces a predominantly olefinic C.sub.5.sup.+ MOG effluent which is condensed in a high-temperature knock-out drum to provide a "wild" C.sub.10.sup.+ condensate (so termed because it contains a substantial amount of C.sub.9.sup.-), and the uncondensed C.sub.9.sup.- vapors are then debutanized. In the second stage, the bottoms fraction of C.sub.5.sup.+ liquid gasoline range hydrocarbons from the debutanizer, is converted to distillate in the MOD reactor, under higher pressure than in the primary stage, but also at low severity.
In the lubes mode, predominantly olefinic C.sub.4.sup.+ MOG effluent is debutanized and the fractionated C.sub.5.sup.+ gasoline range hydrocarbons are converted to lubes in the MOL reactor (the secondary stage), under higher pressure than in the primary stage, but also at low severity.
The effectiveness of the combination of a fluid-bed MOG reactor upstream of a MODL reactor, preferably a single semicontinuous fixed bed MODL (for economics), derives in part from the discovery that the ZSM-5 type catalyst, used in our continuous regenerative primary-stage process under our "easy" conditions, does not appear to suffer from a sensitivity (poisoning) to basic nitrogen-containing organic compounds such as alkylamines (e.g. diethylamine), or, to oxygenated compounds such as ketones, a proclivity which is characteristic of the catalyst under the process conditions of prior art olefin oligomerization processes, particularly the fixed bed processes operated at high pressure. Such processes require the addition of hydrogen as a preventative antidote. It will be recognized that alkylamines are used in treating light gas streams, and ketones are typically present in Fischer Tropsche-derived light ends streams, both of which streams are particularly well-suited for upgrading by oligomerization. Though our process is not adversely affected by the presence of hydrogen, there is no readily discernible economic incentive for using hydrogen in either the primary-stage or secondary-stage reactors, and we prefer not to do so.
Though the earliest prior art, moderate-pressure processes, for example those disclosed in U.S. Pat. Nos. 3,827,968 and 3,960,978 to Givens et al, used a zeolite catalyst to oligomerize lower olefins under moderate conditions, and produced excellent conversions to distillate range olefins in a fixed bed microreactor, some over-riding problems relating to operating the process economically were not foreseen (see "Conversion of C.sub.2 -C.sub.10 Olefins to Higher Olefins Over Synthetic Zeolite ZSM-5" by W. E. Garwood presented at the Symposium on Advances in Zeolite chemistry before the Division of Petroleum Chemistry, Inc., American Chemical Society, Las Vegas Meeting Mar. 28-Apr. 2, 1962).
The '978 patent discloses that low alpha ZSM-5 and ZSM-11 catalysts not only have reduced activity for cracking n-hexane and other paraffins, but also produce less than 10% by wt aromatics. The runs were made in a fixed bed microreactor, and, at that time, it was not known that the process required the addition of hydrogen to control coke deposition and to prevent poisoning of the catalyst by nitrogen-containing organic impurities. The basic knowledge that low activity ZSM-5 and ZSM-11 type catalysts effectively oligomerized lower olefins, was used to arrive at improvements in "Catalytic Conversion of Olefins to Higher Hydrocarbons" in U.S. Pat. No. 4,456,779 to Owen et al. which discloses oligomerization of olefins in a MOD reactor system of three downflow fixed beds, in series, with intercoolers; and, more recently, in "Conversion of LPG Hydrocarbons to Distillate Fuels or Lubes Using Integration of LPG Dehydrogenation and MOGDL" in U.S. Pat. No. 4,542,247 to Chang et al which discloses fixed beds in a two-stage catalytic process for converting paraffins to olefins which in turn are converted to gasoline and distillate. The first stage reactor is operated under conditions given in U.S. Pat. Nos. 3,960,978 and 4,211,640 to Givens et al. Under these conditions there is a substantial make of aromatics which are undesirable if the effluent from the MOG is to be converted to distillate (aromatics lower the cetane number, among other things).
In the '779 process, multiple fixed bed reactors are used, each operating in the same range of process conditions, and it was essential to dilute the feed to the reactors with both lower alkanes and recycled gasoline, to maintain a controllable exotherm in the bed. To provide the gasoline recycle, the effluent from the operating reactors (a spare reactor is always being regenerated) is debutanized after oligomerization of olefins is completed. Moreover, the fixed-bed processes in both the '247 and '779 patents require the addition of hydrogen for the reasons given hereinabove. Thus, despite operation at as high a pressure as is economically feasible, the use of hydrogen with a high concentration of lower alkanes dictates that the oligomerization be carried out in the gaseous phase, or vapor/liquid phases, thus aggravating both the heat transfer and mass transfer problems. When we use one or more fixed bed MODL reactors, they may operate with the hydrocarbons in the liquid, gas or super-dense phase, the conditions of operation, irrespective of the phase in which the reactor operates, being determined by economics. When we use a fluid bed MODL reactor, it operates in the super-dense phase, as will be explained in greater detail hereinafter.
Because Chang et al first dehydrogenated a paraffinic feed, they typically converted 30-40% of the paraffins to olefins. The feed to the MOG reactor therefore was predominantly C.sub.3 /C.sub.4 paraffinic, as was the effluent from the MOG reactor, since the undehydrogenated C.sub.3 /C.sub.4 paraffins are not oligomerized. Because, after oligomerization in the '247 fixed bed MOG reactor, the effluent still contained a major proportion of C.sub.3 /C.sub.4 paraffins, Chang et al had to separate the paraffins from the olefins in the effluent (so that the separated C.sub.4.sup.- paraffins could be recycled to be dehydrogenated). Since, under their conditions, the make of C.sub.10.sup.+ components was relatively small, they failed to realize the criticality of separating the C.sub.10.sup.+ components before the effluent from the MOG reactor was further oligomerized.
Though neither Owen et al, nor Chang et al, knew it at the time, in practice, a fixed bed requires the addition of a substantial quantity of hydrogen (for the reasons given), which fixed bed nevertheless is far less effective than a fluid bed for the specific purpose of "cleaning up" the MOG effluent. It is this volume of hydrogen which adds to the already large volume of diluents being used as a heat sink, albeit an inefficient one. Nothing in either the '779 or the '247 patents suggests the surprising benefits of operating with a fluid bed in the absence of added hydrogen and fluidized with a feed containing too little alkanes to serve as a significant heat sink, namely less than about 50% by wt, preferably less than 30% by wt.
The earlier references disclosed that the product distribution from an MOGD reactor may be tailored by controlling process conditions, such as temperature, pressure and space velocity. Gasoline (C.sub.5 -C.sub.10) is readily formed at elevated temperature (preferably about 400.degree. C.) and pressure from ambient to about 2900 kPa (420 psia), preferably about 250 to 1450 kPa (36 to 210 psia). Olefinic gasoline could be produced in good yield and may be recovered as a product; or, it could be fed to a low severity, high pressure reactor system for further conversion to heavier distillate-range products. Distillate mode operation could be employed to maximize production of C.sub.9.sup.+ aliphatics by reacting the lower and intermediate olefins at high pressure and moderate temperature. Operating details for typical MOGD oligomerization units are disclosed in U.S. Pat. Nos. 4,456,779 and 4,497,968 (Owen et al); 4,433,185 (Tabak); 4,456,781 to Marsh et al; and U.S. patent application Ser. No. 006,407 to Avidan et al.
None of the foregoing alternatives disclosed the technical and economic difficulties of operating the MODL reactor with a significant proportion of a C.sub.4.sup.- fraction in the feed, coupled with the advantages of feeding a tailored C.sub.5 -C.sub.9 olefinic stream to a MODL reactor. Therefore they failed to suggest using them in combination with processing steps made possible with strategically positioned unit operations, namely the use of a knock-out drum, debutanizer (or depentanizer) and G/D splitter, to provide such a stream; nor does any combination of processing steps in the prior art suggest the benefits which enure to a person using our process.
The combination of a relatively low pressure fluid-bed, primary MOG reactor, and, a higher pressure secondary MODL reactor is unexpectedly effective because the fluid-bed primary stage rids the feed of poisons while operating under "easy" conditions which produce the maximum conversion of olefins to C.sub.5.sup.+ olefins, substantially free of aromatics, economically. Since only the gasoline is to be fed to a fixed or fluid bed MODL reactor, after separation of the "light ends", namely C.sub.3.sup.- or C.sub.4.sup.- components (depropanizing or debutanizing) which may still contain poisons not adsorbed by the catalyst, the benefits of recycling the light ends or incompletely converted olefins, is great compared with the disadvantages of operating the MOG reactor for maximum conversion to gasoline under higher severity conditions. Because the distillate formed in the MOG reactor bypasses the MODL reactor, only gasoline is fed to the MODL reactor, we obtain excellent per pass conversion and selectivity to distillate. Because a substantial portion of the coke formation takes place in the MOG fluid bed, our MODL reactor desirably operates with little coke deposition.
This invention therefore provides either a continuous or semicontinuous process for oligomerizing light gas containing propene, butenes and pentenes, in a MOG reactor, whether fixed, fluid or moving bed, preferably a fluid bed, to produce a substantially C.sub.4.sup.+ stream in the absence of added hydrogen; separating a C.sub.5.sup.+ -rich fraction from the MOG effluent; and, feeding the C.sub.5.sup.+ -rich fraction to the fixed bed, preferably liquid phase, or fluid bed, preferably super-dense phase MODL reactor. A fixed bed MOG reactor may be used when operating with substantially "clean" or poison-free feed; a moving bed may be used when the disadvantage of dealing with its inherent mechanical problems is outweighed by the advantages of better control of fines. The MODL reactor, whether in the distillate mode or the lubes mode, operates with unexpected efficiency because the gasoline feed is essentially free of inerts and poisons.
U.S. Pat. Nos. 4,417,086 and 4,417,087 to Miller teach a two-zone reactor operating in the transport mode where the relative superficial gas velocity is greater than the terminal velocity in free fall. Though the operation of a fluid-bed is illustrated (example 2 in each of the '086 and '087 patents) note that no operating pressure is stated in the former, and that operating pressure in the latter is 10 psig (24.7 psia, 170 kPa). The general disclosure that the processes may be operated at a pressure in the range from subatmospheric to several hundred atmospheres, but preferably 10 bar or less, and most preferably 0 to 6 bar, (see middle of col 6 in '086, and, near top of col 5 in '087) is not so ingenuous as to be meant to apply equally to the fixed bed (example 1 of '086 and '087, each illustrates 34.5 bar, 500 psi) and the 170 kPa fluid-bed.
In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed to the understanding of catalytic olefin upgrading techniques and have contributed improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The '062 patent discloses conversion of olefins to gasoline or distillate in the range from 190.degree.-315.degree. C. and 42-70 atm; and this, and the '640 and '992 disclosures are incorporated by reference thereto as if fully set forth herein.